What's Your IQ On SPF?

Next up, in all this summer heat, what could be better than summer science? And if you're headed out to the town, to the beach, sailing, maybe going for a hike, my guess is you're probably taking along a bottle of sunscreen to protect yourself against that blazing summer sun. But do you know how sunscreen actually works, how it protects your skin from those UV rays? We sent our intern Eli Chen out to Times Square and Bryant Park here in New York to ask that question to a few people getting their rays.

UNIDENTIFIED WOMAN #1: Well I always thought it just kind of coated your skin to try to protect you from the sun.

UNIDENTIFIED MAN #1: Coat your skin, maybe the pores and all, so the sunrays can't get in and...

UNIDENTIFIED MAN #2: Must change the melatonin in the skin?

UNIDENTIFIED MAN #3: I think a sunscreen is just ridiculous.

UNIDENTIFIED WOMAN #2: I know it goes into the bloodstream.

UNIDENTIFIED MAN #4: You know, believe it or not, I actually used to know this because my science teacher explained it to me, but I totally forgot.

FLATOW: Yeah. How many of you can remember? Do you think you can do better? Well, if not, here to remind you how sunscreen works is Dr. Jennifer Linder. She's a dermatologist in Scottsdale, Arizona, where they know something about sun, and an assistant clinical professor in the Department of Dermatology at U.C. San Francisco. Welcome to SCIENCE FRIDAY, Dr. Linder.

DR. JENNIFER LINDER: My pleasure.

FLATOW: First things first, how does sunscreen work?

LINDER: Well, great question. So as everybody knows, we get UV rays from the sun, and in the simplest terms, sunscreen is able to either scatter those wavelengths, or it is able to absorb them and release them as heat. And depending on what type of sunscreen you're using and what the active ingredients are, it works in slightly different ways.

So there's physical blockers, which are inorganic compounds, and those are usually either zinc oxide or titanium dioxide. And they're crystal in structure, and based on their size, they're able to scatter the light and essentially kind of reflect it back. But they also have crystalline structures which can absorb the electrons and take that lattice into a higher energy state. And then as the lattice relaxes, it releases it as energy.

FLATOW: Mm-hmm.

LINDER: And what's interesting is, is that based on the size of the molecule, if it's large, it's actually able to reflect back visible light as well, which is the reason why we used to think about all of our lifeguards having that white pasty stuff...

FLATOW: Right.

LINDER: ...in their noses, and that was because it was reflected back to visible light.

FLATOW: Oh, interesting.

LINDER: But now, we're able to micronize that. And when we do that, we're able to make those molecules smaller, and you can focus just on the UVB and UVA, which is important for sun protection.

FLATOW: Mm-hmm. This is SCIENCE FRIDAY from NPR, talking with Jennifer Linder. You say it's UVA and UVB, make sure you have that in the sun - both - protection from both kinds of ultraviolet.

LINDER: That is correct. It's actually probably the most important thing for people to remember because you want to have broad-spectrum covered. The reason is you can think of the UVB rays as being the burning rays, and you can think of the UVA rays as being the aging rays, and that's because the UVB rays go into the epidermis, which is where the nucleus is for the keratinocytes, the ones that when the DNA is changed, actually it turns into skin cancer, versus the UVA rays are longer wavelengths. And they actually go - capable of going into the deeper dermis, which is where the collagen is. And when you break down collagen, that's how you age, and probably about 80 to 90 percent of aging is actually a result of sun exposure.

FLATOW: What about the SPF factor? Does SPF 30 protect you twice as much as, say, SPF 15 and half as much as 60?

LINDER: Great question. So it's - actually the way that the studies are done it's a little bit confusing because an SPF of 15 protects you from 93 percent of UVB rays, and it's only UVBs that SPF actually tells you about. Now, an SPF of 30 is going to protect you from 97 percent of those UVB rays. And when you get up to 50, you're only protecting yourself from 98 percent. So you can notice it's partial percentage points differences...

FLATOW: Yeah.

LINDER: ...between that 30 to 50. And when you get up to that 100 range, it's even a smaller difference, and that's the reason why the FDA is going to release new guidelines for rules of how we're going to label our sunscreens. And it's going to finally happen this December, so that we're not actually be able to have a sunscreen higher than 50 because it really gives people a false sense of security when they're using those higher numbers when they're really not getting that as much.

FLATOW: Yeah. They think they can stay out forever if they have 100 or something.

LINDER: Right. And that's absolutely not the case.

FLATOW: Tell us how a tan - is a tan protective when you get a tan? Is it - does it have a SPF number?

LINDER: So it does, but it's probably only an SPF of two or three, and that's because the tan is your body putting a little bit of melanin in the skin, which acts as a natural, again, reflector of the sun. But what happens is, is that most people actually continue to get a burn. And if you look closely at your skin when it's tan, you kind of press on it, you'll see there's a little bit of pink underneath there, and that's actually the burn that's happening at the same time. And so getting a pre-tan before you go on vacation actually provides very little long-term protection.

FLATOW: Does the sunscreen block the creation of vitamin D by blocking the sun?

LINDER: It actually does block a little bit, but I think what's important to remember is that you can get vitamin D in three different ways. You can get it from sun exposure. You can also get it from food sources, such as fortified cereals or milk, oily fishes like salmon are a great source. But what I recommend to my patients is to actually get it from oral supplementation.

And we don't know yet what the exact number should be, even what the recommended serum level should be. It's still really being elucidated at, and we study it on a regular basis. But what I think we can feel comfortable with is recommending 1,000 international units a day to people from oral supplementation.

FLATOW: You see all these ads for clothing with sunblock built in, wearable sunscreen and things like that. Is that possible? Is clothing - can it be protective that way?

LINDER: It is, and it's a fantastic source of protection because one never wants to rely just on sunscreen alone. The easiest way to tell how protective material is is to actually hold it up and look through at the sun and see how much sunlight is getting through. So a pair of jeans will have an SPF of probably like 2,000 because it's such a thick weave. Versus a light-cotton T-shirt, if you hold it up, lots of sunlight goes through, and you can actually get a sunburn pretty easily through a light-cotton T-shirt. So a heavier weave and darker colors actually going to provide additional protection.

But the other thing that's pretty cool now is that they have new types of additives they can add to materials that have natural sun protection characteristics to them, and you can buy clothing that is sun protected. And you can even buy a detergent that you can wash your clothes in that lasts for multiple cycles, somewhere between like 10 to 30, that actually adds sun protection to the clothes.

FLATOW: Dr. Linder, thank you very much for the ABCs of sun protection.

LINDER: My pleasure.

FLATOW: Jennifer Linder is a dermatologist in Scottsdale, Arizona. We're going to take a quick break, and we're going to come back to barbecue science. We'll talk summer science but - all the mythology that goes on when you eat outside. We're going to talk about it so stay with us. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR. Transcript provided by NPR, Copyright NPR.